The antonyms for the word "most mirthless" would be cheerful, joyful, happy, jolly, lively, hilarious, amusing, entertaining, and fun. These words represent the opposite of the feeling of being mirthless or joyless. Being cheerful and joyful can bring positivity and happiness to one's life. Being entertained and amused can give a sense of excitement and thrill. Jolly and lively can bring a sense of energy and enthusiasm within oneself. Thus, antonyms of "most mirthless" are the words that represent happiness and a positive outlook towards life.